Question: Which of these numbers is prime? ${22,\ 43,\ 55,\ 57,\ 81}$
A prime number has exactly two factors: 1 and itself. The factors of 22 are 1, 2, 11, and 22. The factors of 43 are 1 and 43. The factors of 55 are 1, 5, 11, and 55. The factors of 57 are 1, 3, 19, and 57. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 43 is a prime number.